7.4 Reinforcing Steel, Splices & Post-Tensioning

7.4 Reinforcing Steel, Splices & Post-Tensioning

Reinforcing Steel: Standards, Sizing & Properties

Reinforcing steel provides high tensile, flexural, and shear resistance to overcome concrete's inherent weakness in tension (concrete tensile strength is only ~10% of its compressive strength).

ASTM Rebar Specifications

• ASTM A615 (Billet-Steel Deformed Bars): Standard structural reinforcing bars. Most common is Grade 60 (minimum yield strength $f_y = 60,000 psi$) and Grade 80 ($f_y = 80,000 psi$). Not recommended for field welding without strict chemical analysis.
• ASTM A706 (Low-Alloy Steel Deformed Bars): Specifically manufactured for enhanced ductility and field weldability (carbon equivalent $CE <= 0.55%$). Mandatory for seismic-force-resisting systems and structural members requiring welded rebar splices.

Standard U.S. Rebar Sizing System

Under ASTM specifications, bar sizes #3 through #8 designate the bar diameter in eighths of an inch (1/8"). For bar sizes #9 through #18, the number corresponds to equivalent cross-sectional areas of legacy square bars:

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ACI 318 Clear Concrete Cover & ACI 117 Placement Tolerances

Clear concrete cover is the minimum distance between the outer surface of embedded reinforcing steel (including stirrups, ties, and spirals) and the nearest exterior surface of the hardened concrete. Clear cover protects reinforcing steel against fire, moisture penetration, carbonation, and chemical corrosion.

                    ACI 318 MINIMUM CLEAR CONCRETE COVER
                    
  ┌─────────────────────────────────────────────────────────────────┐
  │ 3.0 INCHES (75 mm)                                              │
  │ Concrete cast DIRECTLY AGAINST and PERMANENTLY EXPOSED TO EARTH │
  │ (Unformed footings, mat foundations, deep trench foundations)   │
  └─────────────────────────────────────────────────────────────────┘
  
  ┌─────────────────────────────────────────────────────────────────┐
  │ 2.0 INCHES (50 mm) for #6 through #18 bars                      │
  │ 1.5 INCHES (40 mm) for #5 bars and smaller                      │
  │ Formed concrete EXPOSED TO EARTH OR WEATHER                     │
  │ (Formed foundation walls, exterior columns, exterior beams)     │
  └─────────────────────────────────────────────────────────────────┘
  
  ┌─────────────────────────────────────────────────────────────────┐
  │ 1.5 INCHES (40 mm)                                              │
  │ Beams, girders, and columns NOT EXPOSED TO WEATHER/EARTH        │
  │ (Primary longitudinal reinforcement, ties, and stirrups)        │
  └─────────────────────────────────────────────────────────────────┘
  
  ┌─────────────────────────────────────────────────────────────────┐
  │ 0.75 INCH (20 mm)                                               │
  │ Slabs, walls, and joists NOT EXPOSED TO WEATHER/EARTH           │
  │ (#11 bars and smaller in interior suspended slabs/partitions)   │
  └─────────────────────────────────────────────────────────────────┘

Rebar Support Accessories & ACI 117 Tolerances

• Bar Supports: Reinforcing steel must be rigidly supported and tied using non-corrosive plastic chairs, plastic-tipped wire bolsters, or precast concrete blocks (dobies) made of equal-strength mortar with embedded tie wires. Sits on grade require solid-base chairs or dobies to prevent sinking into the subgrade.
• Placement Tolerances (ACI 117):
  • Clear cover tolerance for member depths $<= 12 inches$: $-3/8 inch to +1/2 inch$;
  • Clear cover tolerance for member depths $> 12 inches$: $-1/2 inch to +1 inch$;
  • Longitudinal location of bends and bar ends: $+/- 2 inches$ (except at discontinuous ends where tolerance is $+/- 1/2 inch$).

Rebar Splicing Mechanics & Welded Wire Reinforcement (WWR)

Because standard reinforcing bars are manufactured in fixed stock lengths (typically 20, 40, or 60 feet), bars must be spliced to ensure continuous structural load transfer.

Tension Lap Splices (ACI 318 Section 25.5)

Lap splices transfer tensile stress from one bar to the surrounding concrete paste via bond and bearing on bar deformations, which then transfers stress into the adjacent overlapping bar:

• Development Length ($l_d$): The minimum embedment length required to develop the design yield strength ($f_y$) of the reinforcing bar without pullout or concrete splitting.
• Class A Lap Splice: Lap length equals $1.0 \times l_d$. Permitted only when the area of steel provided is at least twice that required by analysis over the entire length of the splice, and $<= 50%$ of the total reinforcement is spliced within the required lap length.
• Class B Lap Splice: Lap length equals $1.3 \times l_d$. Standard across most structural designs where more than 50% of the bars are spliced at the same location or where steel is utilized at full capacity.
• Practical Lap Length Range: Tension lap splices typically require 30 to 50 bar diameters (e.g., #4 bar requires ~20 to 25 inches; #8 bar requires ~40 to 52 inches).
• Large Bar Splicing Restriction: Under ACI 318, #14 and #18 bars cannot be lap spliced in tension due to extreme stress concentrations. They must be connected using full mechanical splices or full penetration butt welds.

                       TENSION LAP SPLICE DETAIL
                       
        ═══════════════════════════════════════════╗
                                                   ║ (Bar #1)
                                ◄── Lap Length ──► ║
                                                   ╚════════════════════════
        ════════════════════════╔
        (Bar #2)                ║ Clear Spacing ≥ 1.0" or 1 Bar Diameter
                                ╚═══════════════════════════════════════════

Mechanical Splices & Welded Wire Reinforcement (WWR)

• Mechanical Splices: Proprietary couplers (threaded sleeves, shear-screw couplers, or cold-swaged steel sleeves) connecting two rebar ends. Under ACI 318, a Type 1 mechanical connection must develop at least $125%$ of the specified yield strength ($1.25 f_y$) of the bar; Type 2 connections must develop both $1.25 f_y$ and the specified ultimate tensile strength ($f_u$) for seismic applications.
• Welded Wire Reinforcement (WWR / WWF - ASTM A1064): Prefabricated grid of cold-drawn high-strength steel wires resistance-welded at intersections. Designated by wire spacing and wire cross-sectional area:
  • Designation Example: $6\times6-W2.9/W2.9$ denotes longitudinal and transverse wire spacing of 6 inches on center, utilizing smooth wire ("W") with a cross-sectional area of $0.029 sq in$ (approx. 10-gauge wire). Deformed wire is designated by "D" (e.g., D4.0).
  • Splicing WWR: Sheets must be lapped such that the two outermost cross wires of each sheet overlap by at least 2 inches plus one full grid spacing (minimum 6 inches) and are securely wire-tied.

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Post-Tensioning Systems (PT)

Post-tensioning is a method of prestressing concrete wherein high-strength steel tendons are tensioned against the hardened concrete member after it has achieved sufficient compressive strength. PT introduces active compressive stresses that counteract in-service tensile and bending forces, allowing for thinner floor slabs, longer clear column spans, and virtually crack-free slab-on-ground foundations.

Post-Tensioning Materials & Hardware

• Prestressing Tendons (ASTM A416): High-strength, 7-wire stress-relieved or low-relaxation carbon steel strands with an ultimate tensile strength ($f_{pu}$) of Grade 270 ($270,000 psi$). Standard strand nominal diameters are 0.500-inch (cross-sectional area $A = 0.153 sq in$) and 0.600-inch ($A = 0.217 sq in$).
• Unbonded Mono-Strand System: The standard system for building slabs. The 7-wire strand is continuously coated with corrosion-inhibiting PT grease and encased in an extruded, seamless high-density polyethylene (HDPE) plastic sheathing. The strand moves freely inside the plastic sheathing, transferring prestress forces to the concrete entirely through the end anchorages.
• Anchorages:
  • Dead-End (Fixed) Anchor: Embedded internally within the slab at the far end of the tendon; factory pre-seated with locking wedges.
  • Live-End (Stressing) Anchor: Positioned at the slab edge inside a temporary plastic pocket former where the hydraulic stressing jack is attached.
  • Two-Piece Conical Wedges: Heat-treated steel wedges with internal serrated teeth that bite into the strand as tension is locked off.

               UNBONDED MONO-STRAND POST-TENSIONING ASSEMBLY
               
    Live-End Anchor           Extruded HDPE Plastic Sheathing
    ┌─────────────┐        ┌──────────────────────────────────────┐
    │ [Stressing] │        │  Corrosion-Inhibiting PT Grease      │
    │  Anchor     │════════│ ┌──────────────────────────────────┐ │═════► [Dead-End]
    │   Pocket    │        │ │ 7-Wire Grade 270 Steel Strand    │ │       Embedded
    │   Former    │        │ └──────────────────────────────────┘ │        Anchor
    └─────────────┘        └──────────────────────────────────────┘

Hydraulic Stressing Operation & Elongation Verification

1. Pre-Stressing Concrete Strength: Tendons must never be stressed until field-cured concrete test cylinders achieve the minimum compressive strength specified by the structural engineer—typically $2,500 to 3,000 psi$ (or $approx 75%$ of $28-day f'_c$), reached in 48 to 72 hours.
2. Tensioning Operation: A calibrated hydraulic stressing jack pulls the live-end strand to an initial seating force, then tensions it up to $80%$ of ultimate tensile strength ($0.80 f_{pu} = 216 ksi$). Once target pressure is attained, hydraulic pressure is released, allowing the wedges to seat into the anchor cavity with a power seating seating loss of approximately 1/4 inch.
3. Elongation Verification ($pm 7%$ Tolerance): General contractors must measure and record the physical elongation ($Delta L$) of each strand to the nearest 1/8 inch. The measured field elongation must match the engineer's calculated theoretical elongation within a strict tolerance of $+/- 7%$:

$Delta L = (P * L) / (A * E)$

Where:

• $P$ = Average prestressing force along tendon (lbs);
• $L$ = Total tendon length (inches);
• $A$ = Cross-sectional area of strand ($0.153 sq in$ for 0.5" strand);
• $E$ = Modulus of elasticity of prestressing steel (approx. $28,500,000 psi$).

Critical Safety & Quality Rule: If the measured elongation differs from the calculated elongation by more than $+/- 7%$, the stressing technician and contractor must SUSPEND WORK IMMEDIATELY. Do not cut the tendon tails. The discrepancy must be reported to the Structural Engineer of Record (EOR) to investigate friction binding, wedge slippage, or gauge calibration issues.

1. Strand Tail Trimming & Pocket Grouting: Once elongation records are signed and approved by the special inspector/EOR, excess strand tails extending beyond the live-end anchor are cut using a plasma torch or mechanical hydraulic shear (oxyacetylene flame cutting is prohibited if heat might damage the hardened wedges). The anchor pocket is coated with bonding agent and packed solid with non-shrink, non-metallic cementitious grout to prevent water intrusion and corrosion.